In a Long Term Evolution (LTE) system, a downlink can support 2 codeword spatial multiplexing and 4 antenna ports at most, correspondingly, the number of layers may be 1, 2, 3 and 4; The modulation symbols for each of the codewords are mapped onto one or several layers in the following manner:
assuming modulation symbols for a codeword q are d(q)(0), . . . , d(q)(Msymb(q)−1), wherein Msymb(q) represents the number of symbols obtained by modulating the codeword q; assuming the modulation symbols for the codeword mapped to a layer ν are x(ν)(0), . . . , x(ν)(Msymblayer−1), wherein Msymblayer represents the number of modulation symbols per layer. In this text, the same character represents the same meaning.
For transmission over a single antenna port, a single layer is used, and the mapping manner is as follows:x(0)(i)=d(0)(i), wherein Msymblayer=Msymb(0).
For layer mapping of spatial multiplexing, the mapping manner is shown as in Table 1:
TABLE 1Number ofNumber ofCodeword-to-layer mappinglayerscodewordsi = 0, 1, . . . , Msymblayer − 111x(0) (i) = d(0) (i)Msymblayer = Msymb(0)22x(0) (i) = d(0) (i)Msymblayer = Msymb(0) =x(1) (i) = d(1) (i)Msymb(1)21x(0) (i) = d(0) (2i)Msymblayer = Msymb(0)/2x(1) (i) = d(0) (2i + 1)32x(0) (i) = d(0) (i)Msymblayer = Msymb(0) =x(1) (i) = d(1) (2i)Msymb(1)/2x(2) (i) = d(1) (2i + 1)42x(0) (i) = d(0) (2i)Msymblayer = Msymb(0)/2 =x(1) (i) = d(0) (2i + 1)Msymb(1)/2x(2) (i) = d(1) (2i)x(3) (i) = d(1) (2i + 1)
In the Table 1 above, the number of layers is smaller than or equal to the number of the antenna ports for transmitting physical channels; a single codeword is mapped to two layers only when the number of antenna ports is 4.
The transmission diversity mapping of a single codeword-to-layer is shown as in Table 2:
TABLE 2NumberofNumber ofCodeword-to-layer mappinglayerscodewordsi = 0, 1, . . . , Msymblayer − 121x(0) (i) = d(0) (2i) x(1) (i) = d(0) (2i + 1)Msymblayer = Msymb(0)/2 41x(0) (i) = d(0) (4i) x(1) (i) = d(0) (4i + 1) x(2) (i) = d(0) (4i + 2) x(3) (i) = d(0) (4i + 3)                                          M            symb            layer                    =                      {                                                                                                      M                      symb                                              (                        0                        )                                                              /                    4                                                                                                              if                      ⁢                                                                                          ⁢                                              M                        symb                                                  (                          0                          )                                                                    ⁢                                                                                          ⁢                      mod                      ⁢                                                                                          ⁢                      4                                        =                    0                                                                                                                                          (                                                                        M                          symb                                                      (                            0                            )                                                                          +                        2                                            )                                        /                    4                                                                                                              if                      ⁢                                                                                          ⁢                                              M                        symb                                                  (                          0                          )                                                                    ⁢                                                                                          ⁢                      mod                      ⁢                                                                                          ⁢                      4                                        ≠                    0                                                                                                                                              if              ⁢                                                          ⁢                              M                symb                                  (                  0                  )                                            ⁢                                                          ⁢              mod              ⁢                                                          ⁢              4                        ≠            0                    ,                      two            ⁢                                                  ⁢            null            ⁢                                                  ⁢            symbols                                                        correspond          ⁢                                          ⁢          to          ⁢                                          ⁢                      d                          (              0              )                                ⁢                                          ⁢                      (                                          M                symb                                  (                  0                  )                                            -              2                        )                    ⁢                                          ⁢          and                                                          d                          (              0              )                                ⁢                                          ⁢                      (                                          M                symb                                  (                  0                  )                                            -              1                        )                                 
It is defined, in the demand research report TR 36.814v0.1.1 of LTE-Advanced proposed in September 2008, that LTE-Advanced downlink spatial multiplexing can support transmission of 8 layers at most, therefore, it is still needed to design a solution of mapping to the layers which are more than 4, however, at present there is still not relevant solutions yet.